1. Field of the Invention
This invention relates to a method for the production of an extremely thin film layer of single crystal in a SOI (silicon on insulator) substrate and more particularly to a method for thinning to extremity a single crystal silicon layer bonded to the upper surface of a dielectric substrate and a method for controlling the thickness of the film layer.
2. Description of the Prior Art
As measures to obtain such an extremely thin film layer of single crystal silicon on a dielectric substrate as described above, the following methods have been proposed to date. A first method is the so-called SOS (silicon on sapphire) method which comprises effecting epitaxial growth of a thin film layer of single crystal silicon on a dielectric substrate such as, for example, a single crystal sapphire substrate.
A second method is the so-called SIMOX (separation by implantation oxygen) method which comprises implanting oxygen ions in a single crystal silicon substrate by the use of an ion implanting device and then annealing the oxygen ion-implanted substrate thereby forming an oxide film layer in a part of a prescribed depth within the single crystal silicon substrate.
A third method is the so-called ZMR (zone melting recrystallization) method which comprises forming a thermal oxide film on the surface of a single crystal silicon substrate, then coating the thermal oxide film with a polycrystalline or amorphous silicon film, irradiating the silicon film surface in strips in a direction with an energy beam such as, for example, an electron beam or a laser beam and, at the same time, moving the direction of the irradiation at the right angle thereby melting and solidifying the silicon film and converting the silicon film into a single crystal film throughout the entire surface of the substrate. Though these methods are useful as ways to obtain a thin film layer of single crystal silicon on a dielectric substrate, they invariably have a drawback that the thin film layers of single crystal silicon produced thereby copiously contain crystallographic defects (T. D. Stanley and P. K. Vasudev, Solid State Technology, November 1990, p. 58).
A fourth method is the bonded SOI method. The substrate of the bonded SOI configuration is obtained by preparing two single crystal silicon wafers subjecting at least one of the wafers to an oxidizing treatment thereby forming a silicon oxide film on the surface of the wafer subjected to the oxidizing treatment, superposing the two single crystal silicon wafers one on top of the other in such a manner as to interpose the oxide film between the superposed wafers, subsequently heating the superposed wafers to a prescribed temperature enough to induce bonding thereof, and polishing the wafer on the upper side thereby effecting required thinning. This method, by far, excels the other three methods cited above in respect that the thin film layer of single crystal silicon in the bonded SOI configuration has resulted from polishing a single crystal silicon wafer and, therefore, equals the wafer yet to be polished for thinning in crystallinity. By the technical standard currently available, the production of a thin film layer of uniform thickness not exceeding 1 .mu.m is very difficult on account of accuracy of thinning by polishing.
The SOI configuration incorporating therein a very thin film of single crystal silicon of not more than 500 nm has been attracting attention as a product promising to find utility in finely designed high-speed CMOS IC's. Particularly, the 1G DRAM needs to use the SOI configuration incorporating therein an extremely thin layer of single crystal silicon 100 nm in thickness. Such an extremely thin film layer mentioned above is required to be formed with the thickness thereof controlled so accurately that the fluctuation of thickness may be within .+-.10% of the average thickness. This is because the fluctuation of thickness in the extremely thin single crystal silicon layer has a serious effect of degrading the electrical characteristics of the component elements formed in the layer in terms of uniformity.
The fluctuation of thickness of the thin film layer of single crystal silicon of the bonded SOI configuration attainable by the current thickness controllability of polishing is within .+-.0.3 .mu.m at most over a wafer of a diameter of 150 mm. The best improvement expected to be achieved in near future will be on the order of .+-.0.1 .mu.m. In the case that the average thickness of the thin film of single crystal in the SOI silicon substrate attainable by the current level of polishing is 0.50 .mu.m, it is only fair to consider that this thin film layer has the largest thickness of about 0.80 .mu.m and the smallest thickness of about 0.20 .mu.m. The difference, 0.60 .mu.m, in this case exceeds the average thickness.
An attempt to produce an SOI substrate incorporating therein a thin film layer of single crystal silicon having an average thickness of not more than 0.30 .mu.m may possibly result in complete loss of part of the thin film of single crystal in the course of the work of polishing.
Our previous invention relating to a method for uniformizing the thickness of a thin film of single crystal silicon in the bonded SOI configuration by the use of the action of chemical vapor-phase corrosion excited by the ultraviolet light was already filed for patent. This previous invention aims to attain highly accurate control of the fluctuation of thickness of the thin film layer of the bonded SOI configuration by fictitiously in the memories of a computer system or something like that dividing the surface of the thin film into minute sections of a prescribed area, measuring the thickness of the thin film layer of single crystal silicon in each of the minute sections in advance of the chemical vapor-phase corrosion, placing the thin film layer in an atmosphere of chlorine, fluorine or the compounds thereof as the reactant gas for the reaction of corrosion, irradiating the minute sections of the surface with the doses of the ultraviolet light from a lamp or an excimer laser as the light source respectively adjusted so that the thin film layer of single crystal silicon acquires a single equal thickness in all the minute sections after completion of the chemical vapor-phase corrosion, causing the ultraviolet light to decompose fluorine or chlorine molecules or the compounds thereof thereby inducing generation of active species of fluorine or chlorine radicals or molecules containing such atoms which are capable of exerting the action of a chemical vapor-phase corrosion on silicon, and consequently effecting required thinning of the single crystal silicon layer by the reaction of chemical vapor-phase corrosion. In the actual uniformization of the thin film layer of single crystal silicon, the uniformity of thickness of the thinned film layer can be enhanced by repeating several times the measurement of the thickness of thin film layer in the imaginary divided minute sections and the reaction of chemical vapor-phase corrosion.
The methods available for the measurement of thickness of the thin film layer of single crystal silicon are broadly divided under two types. A first method carries out the measurement in a wafer which has been taken out from the vessel used for the reaction of chemical vapor-phase corrosion. This method allows use of just about any instrument available for the measurement of the thickness of the thin film layer of single crystal silicon on a silicon oxide film. It also allows freedom of choice between moving the instrument and moving the substrate itself in altering the position of measurement to a multiplicity of points on the substrate.
Moreover, the technique capable of performing this measurement at a rate of even less than 1 second per point of measurement has been already realized. Thus, this method enjoys merits in point of versatility and expeditiousness of measurement. Since this method necessitates taking out of the substrate from the vessel for the reaction of chemical vapor-phase corrosion, it entails the disadvantage that the inevitable displacement of the entrapped gas in the reaction vessel consumes time.
A second method performs the measurement of the thickness of the film layer of single crystal silicon on the substrate, with the substrate left standing within the vessel for the reaction of chemical vapor-phase corrosion. While this method has an advantage of obtaining the measurement without requiring taking out of the substrate from the reaction vessel, still it has a disadvantage that the use of the measuring instrument outside the reaction vessel results in an inevitable elongation of the distance of measurement between the sensor and the sample and the movement of the measuring instrument or that of the substrate required for the alteration of the position of measurement is liable to jeopardize the accuracy of measurement. When a mechanism for moving the substrate is installed within the reaction vessel or the measuring instrument is accommodated in the reaction vessel for the sake of this method, another disadvantage arises that the apparatus gains unduly in size and complexity.